In many organic compounds, such as linear alkylbenzenes, trace amounts of unsaturation can cause undesirable by-products in downstream reactions. Since the exact nature of the unsaturation may vary and may even be unknown, indirect methods of measuring unsaturation are typically used. One well-known method of measuring trace unsaturation is the Bromine Index and is based on the fact that any chemical that can react with bromine at 0° C. in less than 15 minutes will cause an increase in Bromine Index. The measurement of Bromine Index is described in detail in ASTM D2710, the entire contents of which are incorporated herein by reference. The Bromine Index indirectly measures the olefin content of aromatic containing hydrocarbon samples using potentiometric titration. Specifically, the Bromine Index is defined as the number of milligrams of bromine consumed by 100 grams of hydrocarbon sample.
Linear alkylbenzenes are used in the commercial production of detergents by a process involving sulfonation. In this process, excessive unsaturation in the linear alkylbenzene can result in undesirable discoloration of the detergent, typically as measured by its Klett color rating. As a result, linear alkylbenzenes used in detergent production are typically required to have a Bromine Index less than 10.
Linear alkylbenzenes are typically produced by contacting benzene with a long-chain (having more than 10 carbon atoms) alpha-olefin in the presence of a supported acid catalyst or a solid acid catalyst. However, this process frequently produces linear alkylbenzenes with Bromine Index values significantly in excess of 10 and sometimes in excess of 50. Thus additional processing to reduce the Bromine Index may be required.
Clays have been used for many years in the commercial treatment of organic products, such as paraffins, benzene, toluene and xylene, to reduce their Bromine Index. However, linear alkylbenzenes (LAB) are much larger molecules with higher molecular weight and the residual olefinic compounds can be highly substituted such that the activation of the olefinic group is sterically hindered. As a result, the treatment of LAB product with clays has resulted in relatively low catalyst activity and short cycle life at low temperatures and excessive cracking activity at higher temperatures resulting in lower yield of LAB after treatment.
More recently, zeolites, and particularly large pore zeolites, have been proposed as replacements for clays in the removal of bromine reactive contaminants from organic feedstocks. Thus, for example U.S. Pat. No. 4,795,550 discloses the use of aluminosilicate zeolites to remove trace olefins from aromatic and naphthenic feedstocks so as to reduce the Bromine Index of the feedstocks from initial values of 50 to 2000 to final values of 0.1 to 50. Among the feedstocks mentioned as suitable in the '550 patent are C16 to C20 linear alkylbenzenes.
U.S. Pat. No. 6,031,144 discloses a two-step process for reducing the residual olefin content of an alkylation reaction product of a single-ring aromatic hydrocarbon with an at least C16 olefin in which at least a portion of a non-alkylated single-ring aromatic hydrocarbon is removed in a first step; followed by a second step in which the remaining reaction product is reacted in the presence of an acidic catalyst to produce a final alkylation reaction product having reduced olefin content.
U.S. Pat. No. 6,169,219 discloses a multi-stage process for producing linear alkylbenzenes in which benzene is alkylated with a C6 to C20 linear olefin in a first alkylation stage with a first fluorided silica-alumina catalyst at a first temperature and then at least part of the first alkylation stage effluent is reacted in a second alkylation stage with a second fluorided silica-alumina catalyst at a second temperature higher than the first temperature. The second alkylation stage converts polymeric by-products in the first alkylation stage effluent to heavy alkylate which is separated from the LAB product in a downstream separation step.
According to the invention, it has now been found that a catalyst which contains zeolite Y catalyst which has an alpha value of 2 to 30 is particularly effective in reducing the Bromine Index of linear alkylbenzene products, especially in the presence of the benzene and n-paraffin impurities conventionally present in the direct effluent of an LAB manufacturing plant. Thus, using the process of the invention, it has been possible to reduce the Bromine Index of a linear alkylbenzene product from in excess of 80 to less than 10 using zeolite Y catalyst, or preferably an ultrastable zeolite Y (USY) catalyst.